Network Working Group A. Yourtchenko Internet-Draft D. Wing Intended status: Standards Track cisco Expires: October 2, 2010 March 31, 2010 A la carte: Announcing the supported transport protocols via DNS draft-yourtchenko-tran-announce-dns-01 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 2, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract While TCP has enjoyed many enhancements over the decades, it is useful to allow applications to use new transports, such as SCTP. It Yourtchenko & Wing Expires October 2, 2010 [Page 1] Internet-Draft Announcing the transports via DNS March 2010 is inefficient to naively probe the server using the new transport protocol. This document proposes a new DNS resource record which provides an efficient way to query which protocols are supported by a server. The presence of this record will allow the client to avoid the excessive probing. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 3. Description . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Contents of RR . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Comparison with SRV records . . . . . . . . . . . . . . . . . . 4 6. The example of a zone file . . . . . . . . . . . . . . . . . . 5 7. Deployment considerations . . . . . . . . . . . . . . . . . . . 5 8. Evaluation of existing resource record types . . . . . . . . . 6 8.1. TXT RR . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.2. WKS RR . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.3. CNAME RR . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 12.1. Normative References . . . . . . . . . . . . . . . . . . . 8 12.2. Informative References . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Yourtchenko & Wing Expires October 2, 2010 [Page 2] Internet-Draft Announcing the transports via DNS March 2010 1. Introduction Many new transport protocols have been developed in recent years DCCP [RFC4340], SCTP [RFC4960], but have seen little deployment on the Internet. There are two reasons for the lack of deployment: IPv4 middleboxes (e.g, NAT, firewall) which block the new protocol and lack of application support for the transport protocol. By running the transport protocols over UDP ( DCCPoUDP [I-D.phelan-dccp-natencap] and SCTPoUDP [I-D.tuexen-sctp-udp-encaps]), or utilizing IPv6, hosts can overcome the problems caused by IPv4 middleboxes. But once a client application supports a new transport protocol a new problem arises: the client application has no choice but to probe the server (and the network) with all of the viable transport protocols. For example, an HTTP client that supports both TCP and SCTP would need to send a TCP SYN and an SCTP INIT and SCTP over UDP until the client learns if both the network path and the server support TCP, SCTP, or SCTP over UDP. This is described in detail in I-D.wing-http-new-tech [I-D.wing-http-new-tech] This specification defines a new DNS resource record (RR) which helps optimize the probing. With the new RR, the server can advertise which transport protocols it supports for a specific application. With that information, the client application only needs to probe the network path for those transport protocols. This helps reduce unnecessary network traffic and, more importantly, prevents degradation of the user experience when a new transport protocol is enabled on the client and is being deployed on servers on the Internet. 2. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Description This proposal introduces the new resource record type, which we denote "XPORT", that allows to get the answer to the question "For a given application-level protocol APP on the host HOST in the domain DOMAIN, which transport protocol should I choose ?". The client that wants to know the preferred transport, would query for this record on the name _app.hostname.example.com, and parse the received reply in order to get the list of the transports that the Yourtchenko & Wing Expires October 2, 2010 [Page 3] Internet-Draft Announcing the transports via DNS March 2010 server(s) can offer for this application. Each of the returned RRs would contain the transport choice along with the preference value for this choice. The smaller values mean more preferred choices. [LATER: the parameters for the transport may also be specified] Subsequently, the client would use the application name and the resolved transport name to query the SRV records to get the actual hostname and port to connect to. 4. Contents of RR The preference is encoded as a single byte value between 48 and 57 - which gives the textual representation in the ASCII format between '0' and '9'. The protocol and its optional parameters are encoded into a comma-separated string. The protocol is the name of the protocol suitable for passing to getprotobyname(), prefixed with "_". +-------+-------+-------+ . . . | pref | proto and parms .... +-------+-------+-------+ . . . The parameters are protocol-specific. When the casing of the parameters is not explicitly specified, they MUST be all-lowercase. The only currently defined parameter is "udp" which means the protocol-specific encapsulation over UDP. The reason for choosing this format is the possibility of easy (ab)use of TXT record in order to hold the same information, in case the new RR is deemed unnecessary. 5. Comparison with SRV records The SRV records allow to answer the question: "What is the hostname and port?" - given the known application, transport, and domain name. The XPORT records allow to answer the question: "What is the transport ?" - given the known application and domain name. The client stack MAY use SRV records subsequently in order to determine the ports and the real hosts that will serve this application. The server application SHOULD also listen on the default transports for that application, to avoid the malfunction of the clients that are not upgraded (for example, for HTTP that would be TCP transport). Yourtchenko & Wing Expires October 2, 2010 [Page 4] Internet-Draft Announcing the transports via DNS March 2010 6. The example of a zone file Borrowing the example of the zone file from SRV RR [RFC2782] specification, we show the relevant part of it in order to hint the clients about the various protocols that can be used for accessing "http://www.example.com", and to hint that "http://newhost.example.com/" is only accessible via SCTP. $ORIGIN example.com. @ SOA server.example.com. root.example.com. ( 1995032001 3600 3600 604800 86400 ) NS server.example.com. NS ns1.provider.example.net. NS ns2.provider.example.net. ; http transport selection - signal to use SCTP over UDP and SCTP, ; and use tcp as a fallback mechanism for the host 'www' _http.www XPORT 5 _sctp,udp XPORT 5 _sctp XPORT 9 _tcp www A 192.0.2.10 ; the newhost's http is only accessible via SCTP, hint about this. ; note that we still need to have the host listening on TCP ; as well to preserve the legacy hosts' operation. _http.newhost XPORT 1 _sctp newhost A 192.0.2.11 7. Deployment considerations The DNS was chosen instead of any other mechanism due to its widespread deployment, and the logical positioning of the protocol within the applications. The incremental nature of the change means that only one side would need to be modified initially (content provider). The unmodified applications would work with the default transport assignments for the application protocols. The separate resource record allows for the separation of this mechanism from the existing ones. However, considering the possibly Yourtchenko & Wing Expires October 2, 2010 [Page 5] Internet-Draft Announcing the transports via DNS March 2010 lower entry costs, one could consider using the TXT record for storing the preference information - the encoding is crafted in such a way that using the TXT record is straightforward. (TBD: should we consider a "magic number" to be put in the beginning of the TXT record to ensure we can verify the usage of TXT record, or the exotic name is enough ?) It may be noted that the extra lookup mentioned in this proposal adds to the latency of the initial connection establishment. However, assuming the transports are not going to change, it should be feasible to use longer lifetimes, and cache the results at the local recursive name server. 8. Evaluation of existing resource record types Introducing of a new resource record type takes time due to the standards process, the availability of the support for the new resource record type in the nameservers code, problems with the nameservers that fail when queried for the resource record types they do not recognize, and so on. This section evaluates how well existing resource records can provide the functionality similar to XPORT. 8.1. TXT RR The authors have considered following the example of RFC4871 [RFC4871] to use a subdomain starting with an underscore. This would result in the following zone file examples: _xport._http.www.example.com in txt ("SCTP,SCTPUDP,TCP") _xport._http.server.example.com in txt ("TCP") This could be extended further to provide weights like the XPORT resource record, for example: _xport._http.www.example.com in txt ("SCTP=5,SCTPUDP=5,TCP=9") While this could be quickly deployed, it was felt too cumbersome. 8.2. WKS RR The WKS resource record [RFC1035] provides functionality similar to the proposal in this document. However, WKS returns an IPv4 address and a bitmask of ports for the explicit transport protocols (e.g. TCP, SCTP), thus is not suitable for IPv6 nor for protocols which lack an IETF protocol assignment (such as tunneling a transport protocol over UDP [I-D.tuexen-sctp-udp-encaps]). Yourtchenko & Wing Expires October 2, 2010 [Page 6] Internet-Draft Announcing the transports via DNS March 2010 8.3. CNAME RR The authors considered an idea of combining the SRV and defining a "catch-all" transport name, say, "_any", and use the SRV records to help determine the transport(s) offered by the server. The "_appname._any.example.com" record would contain multiple CNAME records, say, "_appname._tcp.example.com" and "_appname._sctp.example.com". The latter names would have the valid SRV records for the respective protocols. Consequently, an SRV lookup of _appname._any.example.com would give a CNAME and the corresponding SRV record - and the contents of the CNAME could be analyzed to extract the protocol to use. However, this would not work, because the section 3.6.2 of [RFC1034] allows only one CNAME. 9. Security Considerations This proposal allows to influence the preferred transport for the client by means of DNS. First possibility is attacker preventing the DNS reply from reaching the client. In this case, the client would fallback to the default. However, assuming the attacker can block the message on the way back to the client, it could block the A/AAAA response messages as well, so this path does not add any new attack vectors. Second possibility is for the attacker to spoof the packets in case they are not there. In this case, the attacker could have spoofed the A/AAAA responses as well and direct the client to an address of his choosing. Therefore, this path does not add any new attack vectors either. 10. Acknowledgements Thanks to Bryan Ford and Janardhan Iyengar for the discussions that led to this draft. Thanks to Erik Kline for the review and suggestions. 11. IANA Considerations This document has no IANA actions. 12. References Yourtchenko & Wing Expires October 2, 2010 [Page 7] Internet-Draft Announcing the transports via DNS March 2010 12.1. Normative References [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. 12.2. Informative References [I-D.phelan-dccp-natencap] Phelan, T., "Datagram Congestion Control Protocol (DCCP) Encapsulation for NAT Traversal (DCCP-NAT)", draft-phelan-dccp-natencap-03 (work in progress), November 2009. [I-D.tuexen-sctp-udp-encaps] Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP Packets", draft-tuexen-sctp-udp-encaps-03 (work in progress), December 2009. [I-D.wing-http-new-tech] Wing, D., Yourtchenko, A., and P. Natarajan, "Happy Eyeballs: Successful Introduction of New Technology to HTTP", draft-wing-http-new-tech-00 (work in progress), July 2009. Yourtchenko & Wing Expires October 2, 2010 [Page 8] Internet-Draft Announcing the transports via DNS March 2010 Authors' Addresses Andrew Yourtchenko cisco 6a de Kleetlaan Diegem 1831 BE Phone: +32 2 704 5494 Email: ayourtch@cisco.com Dan Wing cisco 170 West Tasman Drive San Jose CA 95134 USA Email: dwing@cisco.com Yourtchenko & Wing Expires October 2, 2010 [Page 9]